Cytochemical studies suitable for transmission electron microscopy were employed to further characterize the nature of the vacuolization event occurring in chick embryo cells after infection with Rous sarcoma virus. Results of these studies suggest that the vacuoles are probably formed intracytoplasmically and are not products of pinocytosis involving the plasma membrane. Biochemical assays have been employed to investigate alkaline phosphatase levels in transformed cells and their non-transformed counterparts. A decrease in the level of the enzyme activity is found in the transformed cells, specifically in Rous sarcoma virus transformed cells compared with chick embryo cells.